Plasma Circulation

On plasma circulation
The movement of plasma back and forth through the capillary walls and also into the lymph system is vital to the functioning of cells and also to the survival of the cell tissues.

Capillary walls
The walls of capillaries are a single layer of epithelial cells that form a smooth capillary wall. The epithelial cells have small gaps between them where the cells join.

Gaps between epithelial cells
The gaps between these cells allow plasma to flow back and forth through the capillary walls. The capillary walls then function like a siv that has a mesh size that allows only plasma to flow back and forth through the capillary wall with items suspended in it that are small enough to flow together with the plasma through the mesh of the siv. The plasma transports nutrients and oxygen to all the cells in the body and carries away waste products from the cell tissues. The capillaries run through all the cell tissues in the body including vital organs, muscles, brain etc.

Interstitial space
Capillaries run through the cell tissues inside what is called the interstitial space. That is the small space that cells are surrounded by. This interstitial space is filled with interstitial fluid. It is the same plasma fluid that comes through the capillary walls from the inside of capillaries into the interstitial space. This plasma that flows into the interstitial space goes now by the name interstitial fluid. Plasma carries with it nutrients, oxygen and various forms of white blood cells that perform different immune functions in the interstitial space (lymphocytes, monocytes, macrophages). These different white blood cells fulfill different immune functions such as killing virus infected and cancerous cells, clean up cellular debris from dead cells and clean up pathogens (infectious viruses etc.).

Replacing billions of cells per day
Cell tissues need supplies (nutrients) and oxygen to produce energy with which they fulfill their functions and they also need building materials with which they will create new cells by the process of cell division. New cells have to be created to replace the billions of cells that die every day. Different cells in the body have different life spans. Some live hours and some up to 25 years (bone cells). These dead cells create debris in the interstitial fluid. Also the cell metabolism creates waste products that are dumped by the cells into the interstitial fluid.

Plasma flows back and forth through the capillary walls, but how?
Plasma brings supplies into the interstitial fluid for cell metabolism and to build new cells. The cells will absorb these supplies out of the interstitial fluid and create energy to perform their specific functions in the body. The cells in turn dump their metabolic waste products back into the interstitial fluid and the interstitial fluid with the waste products flows through the capillary walls back into the blood stream and is called plasma again. The blood then carries the waste products to the places where the body gets rid of waste products (liver, kidneys, skin etc). So plasma with supplies flows from the capillaries into the interstitial space. Interstitial fluid with waste products and dead cell debris flows AT THE SAME TIME from the interstitial space back into the capillaries. But how is that possible? And what happens to the dead cell debris that is too large in size to fit through the "mesh" of the capillary wall "siv"?

There must be a pressure differential
For plasma to start flowing from the inside of the capillaries into the interstitial space there must be first of all blood flow through the capillaries and there must be larger fluid pressure on the inside of the capillaries than in the interstitial space that surrounds capillaries so that the plasma with all its supplies and white blood cels will be pushed by the higher fluid pressure through the wall of the capillaries into the interstitial space (through the fine mesh siv that forms the wall of the capillaries). How then can the interstitial fluid with all the waste products and dead cell debris flow back into the capillaries at the same time if the fluid pressure in the capillaries is greater than the fluid pressure in the interstitial fluid? Very simple, as we will learn.

Bernoulli with his "Hydrodymamica" to the rescue
We need to understand some basics about fluid dynamics as first published by the Dutch-Swiss mathematician Daniel Bernoulli in 1738 in his book "Hydrodynamica". The here most applicable insight from this book is that when a fluid flows through a hose that the pressure of the fluid in the hose gradually drops with the distant traveled and it drops exponentially when the diameter of the hose is very small. This then explains why a person with a 100 foot long 1/2" diameter garden hose will be very disappointed to have almost no pressure and only a trickle of water coming out of the end of the hose. This is the reason why fire hoses of great length must be of very large diameter and must have a relatively much smaller nozzle opening at the end so that there is enough pressure and water delivery at the end of the hose to have high enough pressure left to spray water over a long distance to put out fires.

Back to capillaries
Capillaries have an extremely small flow diameter and that causes a large drop of pressure between the arterial end of the capillary and the venous side of the capillary. Hydrodynamic blood pressure on the arterial side of capillaries is 35 mm HG (35 mm of mercury column) and is only 10 mm HG on the venous side of the capillary. It is reasonable now to conclude that the pressure of the interstitial fluid in the interstitial space is somewhere between these two values at a more or less constant 22.5 mm HG pressure. It can now be safely concluded that the hydrodinamic pressure in the middle of the capillary will be about this same 22.5 mm HG. That then makes for a higher pressure of the capillary fluid of 12.5 mm HG over the 22.5 mm HG pressure of the interstitial fluid at the beginning of the capillary and gradually declining toward the middle of the capillary to 0 mm HG pressure differential for an average of 6.25 mm HG higher fluid pressure in the capillary than in the interstitial fluid over the first half of the capillary length. This higher pressure in the capillary pushes then the plasma with supplies and oxygen and white blood cells through the capillary walls into the interstitial space. Then from the middle of the capillary the capillary fluid pressure gradually decreases from being equal to the 22.5 mm hg of the interstitial fluid to the 10 mm HG pressure at the end of the capillary, being 12.5 mm HG below the fluid pressure of the interstitial fluid. That results in an average 6.25 mm HG lower pressure of the blood pressure over the half of the capillary length at the venous end of the capillary. That higher pressure in the interstitial fluid pushes the CO2 and waste product laden interstitial fluid back into the capillary.

What turns capillary bloodflow on and off?
Most capillaries in the body are normally closed and they open periodically during the day when concentration of nutrients and oxygen in the interstitial fluid drop and concentration of cell produced metabolic waste products increase beyond certain healthy levels. The main metabolic waste product released by cells into the interstitial fluid is carbon dioxide, CO2. A greater concentration of CO2 in contact with capillary walls increases the flow diameter of capillaries and a decrease of CO2 near capillary walls decreases the capillary diameter. The average flowdiameter of blood capillaries is 7 microns (micrometers, a millionth of a meter). the average size of a red blood cell is a disc of 2 microns thick and 7 microns in diameter. When the flowdiameter of a capillary drops below 7 microns then a red blood cell will get stuck in the capillary and additional red blood cells will pile up like "a roll of coins" to form a "cork" that will close the capillary. With the capillary closed the flow of plasma with oxygen to the interstitial space will stop and the flow of interstitial fluid with carbon dioxide back into the capillary will also stop. The cells will continue to use oxygen and produce carbon dioxide CO2. This will continue until the oxygen is depleted and CO2 in the interstitial fluid is at high concentration. The high concentration of CO2 around the capillary will cause the flow diameter of the capillary to increase and the blood pressure on the arterial side of the capillary will push the red blood cell "roll of coins cork" out of the capillary and restore capillary circulation which in turn will restore plasma flow with oxygen into the interstitial space and return flow at the same time of interstitial fluid with carbon dioxide back into the capillary. What a simple solution that regulates opening and closing of capillaries.

That still leaves two questions
When the plasma from the capillary with all the fresh nutrients and oxygen and white blood cells is pushed by the blood pressure into the interstitial space it will mix with the "dirty" interstitial fluid that is laden with CO2 and waste products and debris from dead cells. It seems now unavoidable that much of this new mixture of clean and dirty will be pushed out with the interstitial fluid back into the capillary at the same time as the new fresh plasma is coming in. That seems not very efficient. Is that actually happening? And what happens with the larger dead cell debris that is too large to be pushed by the interstitial fluid back into the capillary?

First answer
Yes that is indeed happening. Seems like pouring a half bottle of good wine into a half bottle of bad wine and wind up with a full bottle of bad wine. Slightly different system here. The flow of new fresh plasma and the pushing of dirty mixture interstitial fluid back into the capillary will continue as long as the concentration of CO2 in the interstitial fluid has not dropped to the level when it is no longer in a concentration strong enough to stretch the flow diameter of the capillary above the 7 micron that will keep the capillary open. That will then keep the plasma flow going until the CO2 concentration in the interstitial fluid drops to a low level where the capillary diameter will decrease to where the red blood cells will plug the capillary again.

Second answer
Obviously a similar amount of fluid that comes into the interstitial space must also leave that space again. Only about 90% of the volume of new plasma coming into the interstitial space will leave again as interstitial fluid back into the capillary. The remaining 10% leaves as lymph through the lymphatic capillaries. The lymphatic capillaries have their beginnings inside the interstitial space. Lymphatic capillaries also have a wall of endothelial cells but these lymph cell walls have larger openings in them for fluid to flow into with larger debris being able to be pushed into the lymph capillaries. The endothelial walls of lymph capillaries are different in that they allow fluid only to enter into the lymph capillaries but do not allow any flow into the other direction back into the interstitial space. Instead of adjoining endothelial cells like blood capillary cells, the lymph capillaries have overlapping endothelial cells in which the overlaps are pushed open like a one way swinging door by the fluid pressure of the interstitial fluid that is higher than the pressure in the lymph capillaries. Any flow in the opposite direction is impossible because the one way swinging door will slam shut. The door openings of the one way doors are quite a bit larger than the small openings in blood capillaries that allow plasma to flow out of the blood capillaries and for interstitial fluid back into the blood capillaries. These so much larger "door openings" in the walls of the lymph capillaries will allow the largest of dead cell debris and other unwanted matter such as bacteria and viruses into the lymph capillaries from where they are transported first to the so-called lymph nodes. The lymph nodes are little "factories" that break down large pieces of debris and other unwanted matter into smaller pieces that the body can easier dispose of. The lymph system has one way valves that transport the lymph fluid and all foreign matter in one direction through the lymph nodes and finally back into the blood circulation into the veins via the right lymphatic duct and thoracic duct. XXXXXXXXXX

There the lymph joins the plasma in the blood again and starts its next journey through the cycle of plasma circulation.